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Old 06-28-2008, 10:44 AM   #41
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At 70 MPH 70% of your energy losses are due to aerodynamics.

Aero drag is a function of speed squared.

Just use the speed times itself as a comparison number.

30x30=90
60x60=360

Four times the energy loss due to aero drag, from 30 to 60 MPH.

The higher speed the greater the sustained load on your engine and the more efficient it's production of power for the same amount of fuel consumed per horsepower delivered.

Exponentially greater drag vs, more horspeower per unit of fuel consumed.

The point where the drag overcomes the increased efficiency is the speed at which you will get the best mileage.

regards
gary
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Old 06-28-2008, 11:18 AM   #42
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Yeah, aero drag is a function of CdA and speed though, if you get a flow change, your CdA is different so makes a crinkle in the curve.
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Old 06-28-2008, 11:28 AM   #43
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Some relevant theory ...
http://www-mdp.eng.cam.ac.uk/library...sc/node11.html
http://www.eng.fsu.edu/~dommelen/pap..._a/node57.html
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Old 06-28-2008, 01:34 PM   #44
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good basic reads roadwarrior :thumbsup:
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Old 06-28-2008, 01:43 PM   #45
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Heh, dunno if you were joking about "basic" since the second one is probably a bit heavy going...

The first link I thought this was pretty clear in the bottom half of the text..
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A dramatic change takes place when the Reynolds number is around 2x105 when the boundary layer becomes turbulent before separation. Now the separation is postponed since a turbulent boundary layer is able to sustain for a longer time than a laminar flow. The point of separation S now is found at 1300 as shown in Fig.6.10. Notice that the wake has now narrowed. The CP distribution indicates that the pressure in the wake is now higher than that for the laminar case (Fig.4.31A.). The consequence is that CD is now reduced to about 0.3.
But I suppose I should mention that due to typical dimensions of cars the characteristic length, typically taken as about 1.5m the Reynolds number range of highway speeds is between 2x10^5 ish and 3x10^5 ish. Thus as seen for the cylinder there, somewhere between 50mph and 80mph we might see flow transitions, dropping Cd... now I'm looking for it's it's very noticable with my minivan. It's possible though that full size vans might not be able to make it to the speeds this happens, and that small vehicles like the insight might get the transition happening at 50ish.
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Old 06-28-2008, 05:46 PM   #46
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Quote:
Originally Posted by R.I.D.E. View Post
At 70 MPH 70% of your energy losses are due to aerodynamics.

Aero drag is a function of speed squared.

Just use the speed times itself as a comparison number.

30x30=90
60x60=360

Four times the energy loss due to aero drag, from 30 to 60 MPH.

The higher speed the greater the sustained load on your engine and the more efficient it's production of power for the same amount of fuel consumed per horsepower delivered.

Exponentially greater drag vs, more horspeower per unit of fuel consumed.

The point where the drag overcomes the increased efficiency is the speed at which you will get the best mileage.

regards
gary
Drag force is a function of speed squared, but power is a function of speed cubed.
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Old 06-28-2008, 06:38 PM   #47
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Power is irrelevant when you are discussing efficiency.

regards
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Old 06-29-2008, 05:23 PM   #48
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One should never ignore any variables when discussing such a delicate matter like fuel economy in a car.
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Old 06-29-2008, 08:03 PM   #49
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bah. it wasnt a hard read by any means and could be summarized by saying that there is the transition in flow as a speed for a shape is reached. ive always considered something like that but never pursued it.
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Old 06-29-2008, 08:16 PM   #50
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Efficiency is generally sacrificed in order to produce unnecessary power.

When was the last time you drove a really underpowered vehicle.

If you want efficency drive your car like a tractor trailer, with slightly higher engine speeds when shifting.

Honda is developing an engine design that uses 2 plugs and two valves, because its more efficient than 4 valve designs. Smaller valves fewer valves, create higher swirl and more efficient combustion, at the sacrifice of peak power.

Any fool can pack more air and fuel in an engine and make it more powerful. The real science is making it more efficient. I can build a more economical car with a Flathead Ford V8 from the 1930's than you can with a high revving 4 valve per cylinder engine.

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